The visual display of air bubbles rising through water has proven to have a very attractive and pleasing effect, particularly when lighted from above. This display is most often seen in aquariums, where the bubbles serve the primary function of aerating the water. These rising air bubbles have also been utilized to create extra motion in the aquarium to complement the movement of the fish. One type of extra motion that has been used is the small horizontally axial "water wheel", in which bubbles are released under the wheel causing it to rotate. Another effective technique is the use of a hinged "clam shell", positioned over the bubbles, so that when the shell gathers enough air to become buoyant, it will swing open and release a burst of larger bubbles on a periodic basis.